This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the disclosure. Accordingly, it should be understood that this section should be read in this light, and not as admissions of prior art.
Offshore oil and gas production has been conducted from platforms secured to the ocean bottom for many years. In designing such platforms, engineers must understand the environmental forces that result both from offshore winds, waves, and currents, and from earthquakes. The wind, wave, and current storm condition that engineers consider in designing an offshore platform generally involves surface wave energy with a period in the nine to sixteen second range. On the other hand, earthquakes generally involve energy with a period in a range from zero to two seconds. To the extent possible therefore, engineers design offshore platforms with frequency responses outside of these two period ranges. This design focus of the engineering community can be referred to as “isolation,” or “detuning,” of the platform's response from environmental excitation.
Among the types of platforms that have been used in the offshore industry are Steel Piled Jackets (SPJs) and Compliant Towers (CTs). SPJs differ from CTs in the manner of the detuning of environmental energy from the response of the platform. The SPJ, a rigidly-designed structure, typically has a natural period in the approximate range of two to four seconds—substantially below the principal range of storm energy but above the range of earthquake energy. On the other hand, CTs, which are flexibly-designed structures, have a natural period in the approximate range of twenty to thirty seconds—substantially above the principal range of both storm energy and earthquake energy. Generally, SPJs are economically viable structures in water depths less than approximately 1,000 feet, whereas CTs are economically viable structures in water depths greater than approximately 1,000 feet.
The surface facilities of offshore platforms, referred to generally as the topsides or as the decks, are also subject to earthquake energy effects. In particular, the surface facilities of SPJs are subject to earthquake energy effects due to 1) the close relationship between the natural period of SPJs and the period range of earthquake energy; 2) the two part energy amplification to which such SPJ surface facilities are subjected, first via the propagation of the motion through the soil column system and second, through the interaction of the soil system with the SPJ structure; and 3) the further amplification of equipment response through surface facility module vibration. For all these reasons, among others, engineers continually search for mechanisms to isolate surface facilities from earthquake energy.
The earthquake excitation challenge has been previously addressed via methods of isolating the deck from the lower substructure of the SPJ. For example, the paper “Structural platform solution for seismic arctic environments—Sakhalin II offshore facilities”, Clarke, Buchanan, Efthymiou and Shaw, Proceedings of Offshore Technology Conference, Houston, Tex., OTC 17378 (2005), proposes the use of a friction bearing to dynamically isolate the deck of a gravity-based concrete structure. However, the friction bearings depend on vertical load and hence vertical acceleration for effectiveness. This dependence may result in deck uplift, with a consequent risk of toppling or shearing of the deck due to excessive horizontal and vertical accelerations. In addition, surface friction deterioration of the bearings in the marine environment generally requires continuous monitoring and maintenance.
CTs are less significantly influenced by earthquake excitation, due to the nature of their design. CTs yield to excitation energy by oscillating around a bottom underwater section (or base) in a controlled inverted pendulum manner. This oscillation creates an inertial restoring force which opposes the applied forces. That restoring force may also be augmented using one or more alternatives such as guy lines, buoyancy tanks and pile assemblies. See, for example, U.S. Pat. Nos. 4,610,569-A, 4,696,601-A, and 4,696,603-A.
The earthquake-compliant offshore platform disclosed in WO/1998/058129-A is a substantially vertical, space-frame structure extending upwardly from the floor of the body of water to a point located above the surface of the body of water. The platform has foundation means for attaching the space-frame structure to the floor of the body of water and a deck structure attached to the upper end of the space-frame structure. The natural vibration period of the platform is designed to be greater than the primary excitation period of earthquake energy and less than the primary period of storm energy. As noted above, however, such designs are generally only economically feasible in relatively deep water, typically greater than approximately one thousand feet.
The foregoing discussion of need in the art is intended to be representative rather than exhaustive. There remains a need for improved ways of decoupling or isolating the deck of offshore platforms from the energy which results from earthquakes.